1. Field of the Invention
The present invention relates to pulse-echo ranging and more particularly, to a pulse-echo ranging system comprising a first clock generator for generating a first clock at a first clock frequency, a second clock generator for generating a second clock at a second clock frequency slightly lower than the first clock frequency, a burst generator triggered by the first clock for generating burst pulses at the first clock frequency and for conveying the burst pulses to be transmitted to a target, a receiving device triggered by the second clock for generating an intermediate frequency signal by sampling echo pulses reflected from the target at the second clock frequency, and a signal processing device configured to evaluate the intermediate frequency signal to determine the target distance.
The invention further relates to a pulse-echo ranging method comprising transmitting transmit burst pulses to a target at a first repetition frequency, receiving echo pulses reflected from the target, generating an intermediate frequency signal by sampling the received echo pulses at a second repetition frequency slightly lower than the first repetition frequency, and evaluating the intermediate frequency signal to determine the target distance.
2. Description of the Related Art
Pulse-echo ranging systems and methods are known from each of U.S. Pat. No. 7,379,016 B1, U.S. Pat. No. 7,482,971 B2, U.S. Pat. No. 7,633,434 B2, U.S. Pat. No. 7,710,314 B2 and US 2010/0201408 A1.
Pulse-echo ranging systems, such as radar, Time Domain Reflectometry (TDR) or laser ranging systems provide distance or level measurements based on the direct measurement of the running time of microwave or light pulses transmitted to and reflected from a target, e.g., the surface of a fill material in a container.
As the running time for distances of a few meters is in the nanosecond range, a special time transformation procedure is required to enable these short time periods to be measured. Here, the microwave or light pulses are transmitted to the target at a repetition rate or transmit clock frequency that is established by a transmit clock generator. The received echo pulses reflected from the target are sampled at a sampling clock frequency that is slightly lower than the transmit clock frequency. The sampling and a subsequent integration or low-pass filtering leads to an intermediate frequency signal corresponding to the received echo pulses but time-expanded relative thereto by a factor T1/(T1-T2), where T1 is the transmit pulse repetition period and T2 is the sampling period. The time-expansion allows for amplifying, digitizing and further processing of the echo pulses with standard techniques.
Providing the transmit clock frequency and the sampling clock frequency requires a time base having a very fine resolution, high accuracy, linearity and stability, because these factors are directly related to the measurement error.
A digital time base generator having two clock signal generators of slightly different frequencies can benefit from the use of crystal oscillators and phase-locked loop (PLL) circuits that allows the attainment of high accuracy and low jitter. However, during start up of the crystal oscillators, the phase difference between the clock signals is not predictable. If a detector for a zero phase delay is used, the detector must be able to operate in the picoseconds range. Zero phase detector errors may diminish the merits of the digital solution and the measurement time is increased because a waiting time for zero phase detection has to be added.
It is known from U.S. Pat. No. 7,633,434 B2 that the received echo pulses may be sampled in a signal mixer by cross-correlation with sampling pulses having the same shape as the transmitted burst pulses and at the sampling clock frequency slightly lower than the transmit clock frequency. As a result, two pulse shapers are provided for shaping the transmit burst and sampling pulses. Consequently, there may be a significant measurement drift created by different variations over temperature of the propagation delay in the transmit and sampling pulse shapers.
As known from U.S. Pat. No. 7,482,971 B2 or U.S. Pat. No. 7,710,314 B2, the thermal drift can be corrected by switching the transmit pulse into a reference delay. However, this known solution is complex and the additional microwave switch affects the measurement signal.